where do you put the stop leak in the evaporator unit?

Stop leak is introduced into the A/C system via the low pressure service port. FYI, the low side port is smaller than the high side port. In my experience, stop leak works if the leak is small; and, only when the entire A/C system has been put into a state of vacuum to remove ALL moisture. The way the stop leak works is that it reacts with moisture to harden, thereby sealing the leak. The moisture it reacts with, ideally, is outside the system as the stop leak escapes into moisture laden atmosphere it plugs the hole. If you introduce stop leak into an A/C system that contains moisture within the system, the stop leak will harden inside the system and clog passage ways, particularly the orifice tube or expansion valve.